Earlier quoted context omitted.
It's "a fundamental principle of justice" when "justice" is read in the sense of "law". Not necessarily otherwise. Principles of law apply in a court of law, not everywhere.
No. There are many systems of law that are unjust. Just like the behavior you are defending. Any accusation "substantiated" in secret is not substantiated at all.
B) So no case has ever been decided in closed court in your world, or if any have, they're all unsubstantiated? You need to get your mind out of its over-literal legalistic rut, because in the reality the rest of us live in that happens quite frequently, and much of the time we even trust that allegations made in such courts are substantiated even though they aren't made public.
C) This is still not a "system of law" we're discussing. Try to understand the difference.